This invention relates to electrode structures for fluorescent lamps and in particular to an electrode structure comprising a directly heated hollow cathode for use in low pressure fluorescent lamps.
While present fluorescent lamps, typically operating at an efficacy of approximately 70 lumens per watt, are significantly more efficient than incandescent lamps, operating at an efficacy of approximately 15 lumens per watt, nonetheless the efficacy of fluorescent lamps may be improved even further. In particular, it is known that an operating discharge gas pressure within the lamp of approximately 1 torr is most efficacious in terms of the energy efficiency of the lamp. However, present fluorescent lamps operate at a gas discharge pressure approximately 2.5 torr. Moreover, the typical present fluorescent lamp employs a filamentary coil-on-coil cathode structure coated with a conventional emission mix. At the desired low gas pressure, most prior art cathode structures sputter excessively, particularly during lamp starting. This sputtering deposits material on the fluorescent lamp envelope so as to reduce the light output from the lamp and accordingly, shorten its useful life.
Uther known electrode structures include hollow metal cylinders comprising a thin refractory metal material with electron emissive material coating the interior surface of the cylinder. These cylindrical electrodes possess the advantage that diffuse thermionic electron emission occurs during operation of the electrode in a hollow cathode mode which is typically achieved by heating the cylinder to a temperature of approximately 800.degree. C. However, during starting and before this temperature is reached, heavy sputtering in the arc mode of discharge occurs. Thus, electrode structures comprising only a hollow cathode do not survive many lamp starting cycles.
Two U.S. Patents describe certain electrode structures which are relatively insensitive to the problem of sputtering and which are usable in fluorescent lamps having relatively low (that is approximately 1 torr) discharge gas pressure. In the first patent (U.S. Pat. No. 3,883,764 issued May 3, 1975 to Peter D. Johnson and the applicant herein and assigned to the same assignee as the present invention) there is disclosed a cylindrical electrode structure with a filamentary coil disposed within the cylinder. No direct heating of the cylinder occurs however, and the filamentary coil is not in contact with the cylinder. Thus, the hollow cathode portion of the electrode, namely, the cylinder, is heated only indirectly. Additionally, no ohmic or resistive heating of the hollow cathode cylinder itself occurs. In a second U.S. Pat. (No. 4,117,374 issued Sept. 26, 1978 to Harald L. Witting and likewise assigned to the same assignee as the present invention),there is disclosed an electrode structure similar to that discussed immediately above except that the cylindrical structure is tapered so as to speed the transition from an arc discharge mode to a diffuse hollow cathode operating mode. However, heating of the hollow cathode structure itself is only indirectly accomplished and no means for providing direct resistive heating to a hollow cathode structure is disclosed in either of the two aforementioned patents, both of which are incorporated herein by reference as background material.